LT3502 LINER [Linear Technology], LT3502 Datasheet - Page 15

• Manufacturer Part Number: LT3502
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
The minimum operating voltage of an LT3502/LT3502A
application is limited by the undervoltage lockout (3V) and
by the maximum duty cycle as outlined above. For proper
start-up, the minimum input voltage is also limited by the
boost circuit. If the input voltage is ramped slowly, or the
LT3502/LT3502A is turned on with its SHDN pin when the
output is already in regulation, then the boost capacitor
may not be fully charged. Because the boost capacitor is
charged with the energy stored in the inductor, the circuit
will rely on some minimum load current to get the boost
circuit running properly. This minimum load will depend
on the input and output voltages, and on the arrangement
of the boost circuit. The minimum load generally goes to
(6a) LT3502A Typical Minimum Input Voltage, V
(6c) LT3502 Typical Minimum Input Voltage, V
0.001
0.001
7
6
5
4
3
2
1
0
7
6
5
4
3
2
1
0
RUN
0.01
LOAD CURRENT (A)
0.01
LOAD CURRENT (A)
RUN
START
START
0.1
0.1
3502 G19
3502 G21
1
1
OUT
OUT
= 3.3V
= 3.3V
Figure 6
zero once the circuit has started. Figure 6 shows plots of
minimum load to start and to run as a function of input
voltage. In many cases the discharged output capacitor
will present a load to the switcher which will allow it to
start. The plots show the worst-case situation where V
is ramping very slowly. At light loads, the inductor current
becomes discontinuous and the effective duty cycle can
be very high. This reduces the minimum input voltage to
approximately 400mV above V
the inductor current is continuous and the duty cycle is
limited by the maximum duty cycle of the LT3502/LT3502A,
requiring a higher input voltage to maintain regulation.
(6b) LT3502A Typical Minimum Input Voltage, V
(6d) LT3502 Typical Minimum Input Voltage, V
8
7
0.001
0.001
6
5
4
3
2
1
0
8
7
6
5
4
3
2
1
0
RUN
RUN
0.01
LOAD CURRENT (A)
0.01
START
LOAD CURRENT (A)
LT3502/LT3502A
START
OUT
0.1
0.1
. At higher load currents,
3502 G20
3502 G22
1
1
OUT
OUT
= 5V
= 5V
15
3502fc
IN